The present invention relates to a telephone system and more specifically to a transmission level regulating method for a telephone system connectable to both analog and digital communication networks.
As an example of telephone system of this type, there exists a key telephone system connectable to both the integrated service digital network (ISDN) and the (analog) public subscriber telephone network (PSTN). FIG. 1 shows a typical system configuration of this telephone system. On a key service unit 1, there are mounted ISDN interface units 5a and 5b connected to ISDN lines 3a and 3b, a PSTN interface unit 9 connected to a PSTN line 7, extension terminal interface units 15a to 15d connected to lines 13a to 13d from extension terminals 11a to 11d, etc. These interface units are connected to a control unit 20 for controlling the entire key service unit and channel switching, via a data highway 17 serving as a time-sharing transmission line for control data and a speech highway 19 serving as a time-sharing transmission line for speech PCM (pulse code modulation) codes.
Here, there exists a loss of 4 dB on an average between a subscriber for the PSTN line 7 and a terminal office. In addition, some loss is inevitably produced in a 2-4 line switching circuit and a CODEC (coder/decoder) of an office line (PSTN) interface unit 9. Further, in general the key service unit 1 is so designed that there exists a loss of about -1 dB between the office line (PSTN) interface unit 9 and each of the individual telephone set interface units 15a to 15d.
On the other hand, no loss exists between a subscriber for each of the ISDN lines 3a and 3b and a terminal office. Further, the conventional ISDN interface units 5a and 5b perform only signal representation transformation between ternary codes and binary codes, speed conversion between 64 Kb and 2,048 Mb, multiplexing, separation, etc. in general, without performing conversion with respect to the speech PCM codes.
As a result, when one of the extension telephone sets 11a to 11d is connected to an outside line telephone set such as an analog telephone set 23, there arises the following problem: Since both the ISDN and PSTN lines are connected to the key service unit 1, although telephone conversation can be made through any one of the two lines, there exists a difference in speech (transmission) level between the two lines, because the loss through the PSTN line 7 is different from that through the ISDN lines 3a and 3b, as already explained. In other words, the speech level differs according to the used network, when conversation is made with the same person. The similar difference in the speech level occurs between when any one of the extension telephone sets 11a to 11d is connected to a digital telephone set 21 and when any one of the extension telephone sets 11a to 11d is connected to an analog telephone set 23. In addition, the difference in speech level changes being subjected to the influence of installation conditions of the lines 3 a, 3b and 7 and the key service unit 1.
As described above, in a telephone system connected to both the analog and digital communication networks such as ISDN and PSTN, there exists a problem in that the speech (transmission) level differs according to the connected network and further the level difference varies according to the device installation conditions, thus resulting in a difficulty in maintaining an appropriate speech level all the time.